Replace file command with libmagic or code based on it

[TheAssassin]

Follow-up for #590.

I quick-fixed the issue by bundling the file command, but that solution isn't ideal. We should discuss either writing our own code or perhaps build a method using libmagic in order to eliminate the dependency on file.

[probonopd]

Probably using (and bundling) libmagic would be the cleanest solution, but possibly the library expects data files to be in certain absolute paths - how would we bundle these?

[TheAssassin]

See https://linux.die.net/man/3/libmagic and https://linux.die.net/man/5/magic. It is possible to use a non-system database, either by specifying the correct environment variables, or by using magic_load(...).

[probonopd]

magic_load() seems what we should use then. 👍